1. Field of the Invention:
This invention pertains to the art of structural composite materials, or elements of those materials, in particular, fiber forms prepared from non-woven structural fabric. In particular, this pertains to fiber forms wherein at least some of the yarns present are biased at an angle other than 90.degree. with respect to the axis of the fabric, the fiber forms being curved, and the bias of the yarns being maintained over that curve.
2. Background of the Prior Art:
Increasing demands in the transportation industry, including the rail, marine and aerospace fields, coupled with the increasing cost of energy, has placed a high demand on structural materials that are of extreme strength, durability, and at the same time, light weight. Thus, replacements for conventional alloys, and even lighter weight alloys such as aluminum, are constantly being sought. One such group of substitutes includes the composites field, generally including fiber reinforced plastic. One important type of product within this field is the structural article made by infusing or impregnating a non-woven, stitched structural fabric comprised of a plurality of plys of structure yarns (modulus generally in excess of about 6 million, including fiber glass, Kevlar.TM., boron and graphite) with a curable resin, and thereafter molding the product to provide a stiff, light weight finished product. Such non-woven stitched fabrics, and products that may be produced therefrom, are disclosed in U.S. Pat. Nos. 4,416,929, 4,484,459 and 4,550,045, among others. The shaped article to be resin infused is generally known as a fiber form.
Given the disclosure of these references, it is well within the skill of those in the art to prepare articles of simple shape, having straight edges, and no complex configuration or curve. However, where a curved shape is sought, conventional prior art processes used wet, preimpregnated, "tapes", or unitapes, because of the well developed technology concerning the application and lay down of such tapes. Again, in most circumstances, the production of a shape of complex curvature is easily accomplished using such tapes.
However, in certain critical applications, including aerospace applications, it is necessary to include many layers of structural yarn wherein the yarn is biased with respect to the axis of the fabric, generally at angles greater than 30.degree. and in particular .+-.45.degree.. When preimpregnated tapes, or infused fabrics, are employed to prepare curved articles using such biased fabrics, at least two critical problems are encountered. First, owing to the curve in the alignment of the fabric, the bias of the structural yarns is frequently distorted, or destroyed entirely. Although the yarns may end at a given angle, it is not constant along the radius of curvature. Particular applications, such as the preparation of "stringers" and stiffening members for airplanes and the like, require that the bias be constant, uniform and reproducible. The second problem encountered, particularly when using tapes or other wet products, is the tendency of these tapes to buckle or wrinkle when distorted around a curve. Such distortion and wrinkles frequently give rise to voids and flaws in the formed article, rendering them structurely unsound, and unfit for many applications.
Accordingly, there continues to be a pressing need for a curved fiber form with biased yarns, wherein the bias is maintained around the curve at a constant angle, suitable for impregnation with resin and subsequent molding. Similarly, there currently exists no known process for preparing the same.